The present invention relates to a heater control apparatus for a gas concentration sensor.
In an automotive engine, an air-fuel ratio is controlled based on a detection value of a gas concentration sensor. In general, a gas concentration sensor is equipped with a sensing element including a zirconia solid electrolytic substrate. To assure accurate detection of air-fuel ratio (i.e., oxygen concentration) based on the sensing element, it is necessary to maintain the temperature of this sensing element in a predetermined active temperature zone. To this end, a heater is installed in the sensor body. The electric power supplied to this heater is controlled based on a duty ratio.
According to this type of gas concentration sensor, speedily warming up the sensing element is very important to assure accurate operation of the sensor especially when the engine is in a cold start-up condition. However, increasing the temperature so quickly may cause the cracking of sensing element or heater body and also cause the peeling of substrates constituting the sensor.
In view of the above-described problems, the present invention has an object to provide a heater control apparatus for a gas concentration sensor which is capable of assuring adequate warm-up performance free from the cracking of sensor element or heater body and related problems.
In order to accomplish the above and other related objects, the present invention provides a heater control apparatus for a gas concentration sensor, comprising a sensing element including a solid electrolytic substrate and a heater for heating and activating the sensing element. The heater control apparatus of this invention comprises a warm-up heater control means for controlling electric power supplied to the heater based on a control base value being set according to a predetermined duty ratio, when the sensing element is warmed up to an active temperature. A power profile is determined beforehand to set a target heater power. The warm-up heater control means controls the electric power supplied to the heater so as to equalize an actual heater power to the target heater power determined according to the power profile.
More specifically, to quickly activate the sensing element, the control base value is set to a predetermined value (e.g., duty ratio=100%) to promptly supply electric power to the heater. However, to suppress excessive electric power supply to the heater and adopts, the present invention adopts a power profile defining or expressing an ideal transitional change of target heater power, thereby surely eliminating the cracking of sensor element and heater body.
According to a preferred embodiment of the present invention, the warm-up heater control means performs a correction applied to the control base value based on a relationship between a momentary heater power and the target heater power, and controls the electric power supplied to the heater based on a corrected duty ratio.
It is also preferable that the warm-up heater control means performs a feedback control operation based on a deviation of momentary heater power from the target heater power.
It is also preferable that the power profile is converted into map data, and the warm-up heater control means controls the electric power supplied to the heater based on an elapse of time or a cumulative power with reference to map data during a heater control operation.
It is also preferable that the warm-up heater control means corrects the target heater power so as to eliminate a deviation in a relationship between an elapse of time and a cumulative power or eliminate a deviation in a relationship between the target heater power and a momentary heater power.
It is also preferable that the warm-up heater control means limits the electric power supplied to the heater so as to prevent the actual heater power from exceeding the target heater power.
It is also preferable that the power profile is determined under a condition that the duty ratio of the control base value is set to 100%.
It is also preferable that the power profile is determined under a condition that a stationary reference voltage is applied to the heater.
It is also preferable that the power profile is determined under a condition that a stationary reference voltage is applied to the heater, and the warm-up heater control means performs a correction applied to the control base value based on a relationship between a momentary heater voltage and the stationary reference voltage, and controls the electric power supplied to the heater based on a corrected duty ratio. For example, the correction of the warm-up heater control means is performed according to a ratio of the stationary reference voltage to the momentary heater voltage.
The present invention brings the same effects even if the power profile is replaced by an equivalent or comparable profile.
For example, instead of using the power profile, a current profile is adopted beforehand to set a target heater current. The warm-up heater control means controls the electric power supplied to the heater so as to equalize an actual heater current to the target heater current determined according to the current profile. The warm-up performance is adequately maintained. The cracking of sensing element or heater body can be surely prevented.
It is also preferable that the gas concentration sensor is for detecting the concentration of an exhaust gas component emitted from an engine installed in an automotive vehicle. The heater control apparatus receives electric power supplied from a battery mounted on the automotive vehicle. The warm-up heater control means sets a guard value corresponding to a voltage change of the battery to limit the duty ratio of the control base value. This effectively suppresses the excessive power supply to the heater.
It is also preferable that the warm-up heater control means calculates an initial resistance value of the heater and sets a warm-up control time corresponding to the initial resistance value, for performing a warm-up heater control operation during a limited period of time defined by the warm-up control time. For example, when the heater resistance is small, there is a higher possibility that the sensing element may cause a crack. Thus, the warm-up control time is set to a relatively small value.
On the other hand, it is also preferable that the warm-up heater control means enlarges the warm-up control time when an actual voltage applied to the heater is lower than the reference voltage.
It is also preferable that the warm-up heater control means performs a warm-up operation during a limited period of time before starting an ordinary heater power control operation based on a sensing element resistance or a heater resistance.